1. Field of the Invention
This invention relates to the field of measuring impurities in gases and more particularly to a system for measuring impurity levels in noble gases.
2. Description of the Related Art
Noble gases are used in many applications. One example of a noble gas is Xenon, an odorless, colorless, tasteless, nontoxic, monatomic and chemically inert gas. Very small percentages of Xenon gas are normally present in the atmosphere, approximately one part in ten million of dry air. Xenon gas (Xe) is principally shipped and used in liquid or gaseous form and is used in various applications including radiation detection, lasers, ion propulsion, light bulbs, window insulation, medical applications and laboratory research. Xenon gas manufacturers such as, Spectra Gases, Inc., sell standard grades of Xenon gas that are on the order of 99.995% to 99.999% pure, having, for example, from 0.05 parts per million (ppm) up to 1.0 ppm of Oxygen (O2), although slightly higher purity levels are also available.
Another example of a noble gas is Argon (Ar) which is present in the air at a rate of about 1 percent. Argon is readily available in purity levels of up to 99.9995% (research grade), which is fine for welding, plasma jet torches and filling light bulbs. Argon is also used in the production of impurity-free silicon crystals used in the production of semiconductors. As the requirement for larger silicon crystals increases, e.g., for larger integrated circuit die sizes, the demand for higher purity levels increases.
For use in some applications such as certain radiation detectors, gas purity of around one part per billion (ppb) is needed. Among the noble gases, Xenon (Xe) is considered the most difficult to purify, which is unfortunate because it is the most widely used. In many uses of Xenon (Xe), the most damaging contaminants are so-called electro-negative molecules such as oxygen (O2), water (H2O), nitrous oxide (N2O) and carbon monoxide (CO). These molecules inhibit the operation of very sensitive detectors because they trap electrons.
There are known ways to remove impurities such as Oxygen (O2) from a noble gas such as Xenon (Xe). One method of purifying Xenon (Xe) of electro-negative molecules is to initiate a spark between a titanium electrode and a stainless steel electrode that are immersed in the liquid Xenon (Xe). The spark creates titanium dust that readily oxidizes by mixing with any available electro-negative molecules. Methods such as this are known to iteratively produce Xenon (Xe) gas with electro-negative purity levels at or below one part per billion (<1 ppb), but it is very difficult to know when the desired electro-negative purity level is achieved.
Although there are known methods to purify noble gases, it is very difficult to measure the impurities in the noble gas at concentrations of less than one part per million (<1 ppm). One method to measure the electro-negative contamination of liquid Xenon gas is to use the charge deposited by cosmic ray muons. In this, a tank is filled with the Xenon (Xe) liquid under high pressure. Within the tank is an electron drift region formed by a stacked series of rings insulated from one another by ceramic insulators. A negative high voltage is divided by a resistor network, and each tap is applied in order to the conductive rings. Within the tank is a stainless steel anode that is connected to a high-gain amplifier. In our atmosphere, we are constantly bombarded with cosmic ray particles. As each cosmic ray particle hits the Earth's atmosphere, a nuclear reaction occurs which produces pions that decay into muons. When a muon passes through the Xenon gas, it produces an ionized path of electrons and ions. The electrons are drawn by the electric field toward the anode, inducing a signal that is amplified by the high-gain amplifier and, thereby, detected. If the Xenon (Xe) gas was 100% pure, all of the liberated electrons would reach the anode and be detected. The presence of electronegative impurities reduces the number of electrons reaching the anode and, thereby, the amplitude of the induced signal. The average magnitude of the detected signal is measured to determine the estimated purity of the Xenon (Xe). Unfortunately, to achieve a useful measurement, many muons must pass through the Xenon (Xe) gas. Since the rate of interaction of muons within the Xenon (Xe) chamber is typically of the order of one per minute, approximately one half of an hour of measurements is required to make a reliable measurement.
What is needed is a method and apparatus that will quickly and accurately measure the purity of a noble gas.